1. Field of the Invention
The present invention relates generally to maintaining a supply of electrical power to electronic components normally furnished from a line supply of electrical power, and more particularly, to maintaining data stored in volatile memory components for which electrical power has been interrupted or lost.
2. Related Art
Volatile memory components such as a random access memory (RAM) must receive electrical power continuously to maintain the information stored in their memory cells. The bit content of the memory is typically lost whether electric power to the RAM is momentarily interrupted, fluctuates, or is lost.
There are many situations where backup power is required to maintain data in RAM. For example, service interruptions to the line source of power by natural phenomenon, operational problems in the power system, etc. These conditions generally result in a power interruption for a short period of time, perhaps one hour or less. Power backup is also required for momentary power fluctuations which may occur when, for example, heavy loads such as motors, buzzers, or displays are driven. Under such conditions, the backup power source is necessary to maintain a stable source of power. Other situations include providing backup power when the primary power source is turned off. Under these conditions, backup power may be required for an extended period of time, such as one month.
Recently, various approaches have been taken towards providing power backup capability to RAMs to achieve a nonvolatile memory, isolated from the above power interruptions and fluctuations. One of the more common backup power supply arrangements has been to combine a RAM with a dedicated backup battery. Some of these battery backup devices also include a circuit which senses power supply voltage. If this voltage drops below a predetermined battery voltage level, the circuit switches the RAM from receiving power from the line source of power to receiving power from the backup battery. These circuits generally allow current to be drawn from the battery only in the event of a loss of AC power.
There are a number of problems with the above approaches. First, the circuits normally exhibit a time delay before the backup battery is connected to the RAM. This time delay is a result of the turn-on time of the transistors in the circuit. While the delay time is relatively short, the RAM can be exposed to a low supply voltage condition before the battery is connected. This exposure to low supply voltage may cause a loss of data. Some systems have incorporated a capacitor to supply a temporary source of power to the RAM during the switching of power from line voltage to battery.
Another disadvantage of a battery backup power supply is that the charge remaining in nonrechargeable batteries to handle a power outage of unknown duration are difficult to determine. In addition, rechargeable batteries must have a controlled charge rate to limit current to the battery, and periodically requires replacement. This increases the maintenance cost of the system. In addition, the capacitor can supply the necessary voltage for only a limited period of time. Thus, there may be a loss of data should the battery not be replaced at the proper time or is defective for a period longer than the discharge time of the capacitor.
Another disadvantage of battery backup systems relates to the environmental concerns of used battery disposal. Battery backup systems require the implementation of procedures for removing and discharging the battery prior to disposal. Although the use of a capacitor alone to provide backup power would alleviate this environmental problem, capacitors can supply power for only a relatively short period of time, making them ineffective as a long term backup power source.
What is needed, therefore, is a means for backing up volatile memory devices for long periods of time without incurring the problems associated with the use of batteries.